Remove the Python requirement from the CI workflow

Because it conflicted with the newer version now required, and it's not
necessary since the Python version is set in the conda environment.yml.

.github/workflows/tests.yml

@@ -12,7 +12,6 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.7]  #  TODO: Enable additional versions: 3.7
         gcc-version: [8]  #  TODO: Enable additional versions: 6, 7
     env:
       DISPLAY: ":99.0"
@@ -37,14 +36,13 @@ jobs:
       uses: actions/cache@v1
       with:
         path: ~/.cache/pip
-        key: ${{ runner.os }}-${{ matrix.python-version }}-pip-${{ hashFiles('**/setup.py') }}
+        key: ${{ runner.os }}-pip-${{ hashFiles('**/setup.py') }}
 
     - name: Setup Conda
       uses: conda-incubator/setup-miniconda@v2
       with:
         environment-file: environment.yml
         miniconda-version: "latest"
-        python-version: ${{ matrix.python-version }}
         channel-priority: strict
         use-only-tar-bz2: true  # IMPORTANT: This needs to be set for caching to work properly!